Electrophotographic photoreceptor comprising amorphous silicon layer coated with amorphous inorganic material

ABSTRACT

A photoreceptor with a photoconductive layer of amorphous silicon has an overcoating layer to protect the photoconductive layer and to prevent formation of SiO x . The overcoating layer is amorphous C 1-x  X x , Ge 1-x  X x , (BNGe) 1-x  X x  or (BNC) 1-x  X x  where x is greater than or equal to 0 and smaller than 0.5 and X is H, F or Cl.

This is a continuation, of application Ser. No. 134,591 filed Dec. 18,1987, which is a continuation, of application Ser. No. 945,838 filedDec. 23, 1986.

BACKGROUND OF THE INVENTION

This invention relates to a photoreceptor used in electrophotography.

In connection with electrophotographic apparatus such as copyingmachines and document readers, many kinds of materials have beenconsidered for forming a photoconductive layer. Inorganic materials suchas Se, CdS and ZnO as well as organic materials such as PVK-TNF havebeen used previously for this purpose but these previously consideredmaterials generally do not satisfy all the conditions required of aphotoreceptor such as properties related to optical sensitivity,spectroscopic sensitivity, signal-to-noise ratio (light resistance/darkresistance), durability and safety to the human body. It has thereforebeen a common practice to make a compromise to a certain degreeregarding certain aspects and make a choice, depending on the individualcircumstance under which use is contemplated.

Amorphous silicon, with desirable characteristics such as high opticalsensitivity and high durability, has been considered an excellentmaterial. With the conventional photoreceptors of the type having aphotoconductive layer of amorphous silicon, however, use is made of amaterial of amorphous silicon type also for an overcoating layer formedon the surface to protect this photoconductive layer. When ozone isgenerated by the chargers which are used for charging and erasing,silicon in this overcoating layer reacts with ozone to generate SiO_(x)on the image forming surface. Since SiO_(x) has water-absorbingcharacteristics, electric charges on the surface become diffused easilyby absorbed water if humidity is high, causing the image to "run" andthereby adversely affecting the quality of images.

SUMMARY OF THE INVENTION

It is therefore an object o the present invention to provide aphotoreceptor with which generation of SiO_(x) can be prevented on itsimage forming surface and the image can be prevented from running.

The above and other objects of the present invention are achieved byforming an overcoating layer of amorphous C_(1-x) X_(x), Ge_(1-x) X_(x),(BNGe)_(1-x) X_(x) or (BNC)_(1-x) _(X) _(x) (where x is greater than orequal to 0 and less than 0.5 and X indicates a third component such asH, F and C1) on the top surface of a photoconductive layer of amorphoussilicon so as to prevent formation of SiO_(x) on the image formingsurface. If a photoreceptor is formed as described above, theovercoating layer formed according to the present invention serves toseparate ozone, which is generated by the chargers for charging anderasing the image forming surface of the photoreceptor, from silicon inthe photoconductive layer. As a result, SiO_(x) is not generated on theimage forming surface and the images can be prevented from running. Insummary, the present invention serves to improve the image quality of aphotoreceptor and to make amorphous silicon practically usable asphotoconductive material.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate an embodiment of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings,

FIG. 1 is a schematic drawing of a device for the formation of anamorphous silicon layer, and

FIG. 2 is a sectional view of a portion of amorphous layers formed bythe apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

An apparatus for forming an amorphous silicon layer is schematicallyshown in FIG. 1 for the purpose of explaining a method of manufacturinga photoreceptor embodying the present invention. In what follows, aphotosensitive drum of the type used in a copying machine is consideredwith its amorphous layers formed by the glow discharge method.

With reference to FIG. 1, numeral 1 indicates a cylindrical drum ofaluminum which serves as the substrate for the photoconductive layer tobe formed. After its surface is washed thoroughly, the drum 1 is mountedto a support 3 inside a reaction chamber 2. The support 3 is connectedto an external driving means 4 which provides a rotary motion to thedrum 1. After the drum 1 is set, an exhaust valve 5 is opened to removethe air inside the reaction chamber 2 by means of a vacuum pump 6.Numeral 7 indicates an auxiliary valve which is opened to introduce intothe reaction chamber 2 a reactant gas prepared at a desired mixing ratioby means of a mass flow controller 8. The exhaust valve 5 is adjusted inthis process so that pressure is maintained at a predetermined level.After the atmosphere inside the reaction chamber 2 is thus prepared,high frequency power of 13.56 MHz is applied between dischargeelectrodes 9 from a high frequency power source 10. A glow dischargetakes place as a result between the electrodes 9 and an amorphous layeris formed on the surface of the drum 1.

According to the present invention, the apparatus shown in FIG. 1 isused to form three amorphous layers as shown in FIG. 2 on the surface ofthe drum. The first is a blocking layer 11 formed as a foundation layeron the surface of the drum 1 and is made of amorphous silicon. Thesecond is a photoconductive layer 12 for generating and transportingcarriers and is also made of amorphous silicon. The third is anovercoating layer 13 for protecting the photoconductive layer 12 and ismade of amorphous CH according to one embodiment of the presentinvention. The overcoating layer 13 is made of amorphous GeH accordingto another embodiment of the present invention, BNGeH according to stillanother embodiment of the present invention and BNCH according to afurther embodiment of the present invention. Amorphous silicon,amorphous CH, amorphous GeH, amorphous BNGeH and amorphous BNCH of theselayers can be prepared by adjusting the composition of the reactant gasas well as its mixing ratio.

Table 1 shows the conditions under which amorphous layers were formedfor four photosensitive drums embodying the present invention (Examples1-4) and one (Example 5) of prior art type for comparison. The first(blocking) and the second (photoconductive) amorphous silicon layerswere formed under identical conditions for all examples. The third(overcoating) layer of amorphous CH of Example 1 was formed by a glowdischarge by introducing CH₄ gas into the reaction chamber 2. The thirdlayer of amorphous GeH layer of Example 2 was formed by a glow dischargeby introducing a mixture of GeH₄ and H₂ gases into the reaction chamber2. The third layer of amorphous BNGeH layer of Example 3 was formed by aglow discharge by introducing a mixture of B₂ H₆, GeH₄ and H₂ gases intothe reaction chamber 2. The third layer of amorphous BNCH of Example 4was formed by a glow discharge by introducing a mixture of B₂ H₆, C₂ H₆and N₂ gases into the reaction chamber 2. Likewise, the third layer ofamorphous SiCH of Example 5 was formed by a glow discharge byintroducing a mixture of SiH₄ and CH₄ gases into the reaction chamber 2.

                                      TABLE 1                                     __________________________________________________________________________                                             Sub-                                                                          strate                                                                            Layer                            Amorphous                                                                              Flow Rate (SCCM)      Power                                                                              Pressure                                                                           Temp.                                                                             Thickness                        Ex.                                                                              Layer SiH.sub.4                                                                        B.sub.2 H.sub.6                                                                  H.sub.2                                                                          CH.sub.4                                                                         GeH.sub.4                                                                         N.sub.2                                                                          C.sub.2 H.sub.6                                                                  (W/cm.sup.2)                                                                       (Torr)                                                                             (°C.)                                                                      (μm)                          __________________________________________________________________________    1  1     300                                                                              0.1                                                                              500                                                                              -- --  -- -- 0.3  0.5  280 0.6                                 2     300                                                                              0.001                                                                            500                                                                              -- --  -- -- 0.3  0.5  280 30                                  3     -- -- -- 100                                                                              --  -- -- 0.1  0.05 250 0.3                              2  1     300                                                                              0.1                                                                              500                                                                              -- --  -- -- 0.3  0.5  280 0.6                                 2     300                                                                              0.001                                                                            500                                                                              -- --  -- -- 0.3  0.5  280 30                                  3     -- -- 500                                                                              -- 300 -- -- 0.1  0.05 250 0.3                              3  1     300                                                                              0.1                                                                              500                                                                              -- --  -- -- 0.3  0.5  280 0.6                                 2     300                                                                              0.001                                                                            500                                                                              -- --  -- -- 0.3  0.5  280 30                                  3     -- 50 -- --  50 800                                                                              -- 0.1  0.05 250 0.3                              4  1     300                                                                              0.1                                                                              500                                                                              -- --  -- -- 0.3  0.5  280 0.6                                 2     300                                                                              0.001                                                                            500                                                                              -- --  -- -- 0.3  0.5  280 30                                  3     -- 50 -- -- --  800                                                                              50 0.1  0.05 250 0.3                               5*                                                                              1     300                                                                              0.1                                                                              500                                                                              -- --  -- -- 0.3  0.5  280 0.6                                 2     300                                                                              0.001                                                                            500                                                                              -- --  -- -- 0.3  0.5  280 30                                  3      10                                                                              -- -- 500                                                                              --  -- -- 0.1  0.3  250 0.2                              __________________________________________________________________________     *Prior Art                                                               

The photosensitive drums thus prepared were used for copying underdifferent humidity conditions to determine whether images ran (YES) ornot (NO). The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                  Humidity                                                            Example     60%      70%       80%    90%                                     ______________________________________                                        1,2,3,4     No       No        No     No                                      5*          No       Some      Yes    Yes                                     ______________________________________                                         *Prior Art                                                               

For this experiment, each drum was mounted in a copying machine and wasoperated under the following conditions after aging by 10,000 sheets:peripheral speed of the drum=254.4 cm/sec, corona current=90 μA, andbias voltage=164V. The initial surface voltage was 500V and thetemperature of the drum was 31° C. Table 2 shows that images of highquality could be obtained with photosensitive drums embodying thepresent invention in the range of humidity between 60% and 90%. With thephotosensitive drum of prior art type (Example 5), however, there was norun at humidity 60% but the image quality began to deteriorate ashumidity reaches 70% and runs were distinctly visible at humiditythereabove. In summary, it may be concluded that the photosensitivedrums of the present invention are capable of preventing runs and serveto improve the quality of produced images.

If the amorphous CH layer of Example 1 is formed with a high frequencypower source of 0.5 W/cm² or over, uniform copy samples cannot beobtained. If such an amorphous CH layer is analyzed by X-raydiffraction, it is found that minute crystalline structure can beobserved. It may be concluded from this observation that the overcoatinglayer 13 must be of amorphous structure containing no minute crystals.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and many modifications and variations are possible in lightof the above teaching. For example, the amorphous overcoating layer 13is not limited to CH, GeH, BNGeH or BNCH. Hydrogen in these compoundsmay be replaced by or Cl, or may be absent. In summary, the overcoatingamorphous layer according to the present invention may be C_(1-x) X_(x),Ge_(1-x) X_(x), (BNGe)_(1-x) X_(x) or (BNC)_(1-x) X_(x) where X is H, For Cl and x is equal to or greater than 0 and smaller than 0.5.

What is claimed is:
 1. In a photoreceptor including a photoconductivelayer comprising amorphous silicon, the improvement wherein anovercoating amorphous layer is formed over said amorphous silicon layersaid overcoating layer comprising Ge_(1-x) X_(x) where x is equal to orgreater than 0 and smaller than 0.5 and X is an element selected fromthe group consisting of H, F and Cl.
 2. In a photoreceptor including aphotoconductive layer comprising amorphous silicon, the improvementwherein an overcoating amorphous layer s formed over said amorphoussilicon layer, said overcoating layer comprising (BNGe)_(1-x) X_(x)where x is equal to or greater than 0 and smaller than 0.5 and X is anelement selected from the group consisting of H, F and Cl.
 3. In aphotoreceptor including a photoconductive layer comprising amorphoussilicon, the improvement wherein an overcoating amorphous layer isformed over said amorphous silicon layer, said overcoating layercomprising (BNC)_(1-x) X_(x) where x is equal to or greater than 0 andsmaller than 0.5 and X is an element selected from the group consistingof H, F and Cl.